Non-conducted atrial premature beats, i.e. those that do not conduct to the ventricles because of AV nodal refractoriness, may simulate either type I or type II block.
The ECG shown here is from a 70 year old male. There is sinus rhythm with a PR interval that is prolonged (0.34 seconds) but constant. There is a pause between the 3rd and 4th QRS complexes that measures 1.64 seconds, exactly twice the PP (or RR) interval of 0.82 seconds. and there are no sinus P wave during the pause. These features suggest type II 2nd degree SA block. However, the actual cause of the pause is a non-conducted atrial premature beat (arrow) that slightly distorts the ascending limb of the T wave but is otherwise difficult to appreciate. .